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Posted on 09/19/2001 10:43:27 AM PDT by blam

New York subway at risk of major flood

09:52 19 September 01
Ian Sample

Engineers are drawing up plans to prevent the flooding of New York's underground transport network, following the collapse of the World Trade Center. They fear water from the Hudson river could get into the vast seven floor basement and fill underground train tunnels all over the city.

"It could flood a lot of the underground system of New York," says Dan Hahn, of Mueser Rutledge Consulting Engineers, who are working on the disaster site with the New York Port Authority.

The World Trade Center's twin towers were destroyed on 11 September, when hijacked passenger jets were flown into the buildings. Over 5000 people are feared dead.

The basement of the World Trade Center housed seven floors of shops and parking, with an underground PATH (Port Authority Trans Hudson) train station at the bottom. Two train tunnels from the station head west out of the basement, beneath the Hudson river, where they join a network of other train tunnels. Two of these head back under the Hudson and terminate at 33rd Street in midtown Manhattan, where they link with subway stations.

The bathtub

The basement, now full of debris, was built by first cutting four channels, about one metre wide and 21 metres deep into the ground and filling them with concrete. This formed what the engineers call the "bathtub"- a concrete-walled box that reaches down to sturdy bedrock. Soil and rock inside the 65,000 square metre area was then excavated to allow the basement floors to be built.

During building, the walls of the basement were secured against the pressure of surrounding soil and water by anchoring them to the bedrock with steel "tieback" cables. When completed, the floors brace this strain and the tiebacks are cut.

With the basement floors now destroyed or damaged, engineers believe only debris from the building is preventing the basement walls from collapsing in on themselves. If this happens, soil and water from the Hudson will start pouring in.

"If the walls collapse and the bathtub fills with water, it'll go straight out of the tunnels at the bottom," says Aine Brazil, managing principal with structural engineers, Thornton-Tomasetti Group who are working alongside Hahn. "That could send water right up to 33rd Street and into the subway," she says.

Concrete plugs

Engineers say water is already running through the tunnels under the Hudson. Exchange Place, the first PATH station west of the river, is currently under 15 centimetres of water, though this may be from broken water pipes and fire hoses.

To prevent a widespread flood, Hahn has drawn up plans to get into the PATH tunnels that connect with the World Trade Centre site from across the Hudson. Once inside, the tunnels will be filled with huge concrete plugs designed to withstand a 25 metre head of water.

Even so, excavation of the basement - which will not be possible for weeks, according to Brazil - must proceed delicately. Hahn says that for every four to six metres they excavate, they will put in new tiebacks around the walls to support them.

The precaution should also prevent more damage to surrounding buildings, says Brazil. Soil around the walls will shift if the "bathtub" collapses, which could destabilise small or older buildings nearby, she says.

09:52 19 September 01

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